The T1-type pulse code multiplexed (PCM) transmission system commonly employed in the telephone industry uses a pulse format consisting of frames comprising 24 eight-bit segments representing the transmitted information on the 24 channels being multiplexed, plus one extra bit interposed between each 24-segment group. The extra bits are alternately main frame bits and signalling frame bits. They are arranged in a pattern such that the main frame bits alternate between "0" and "1" in even-numbered frames, while the signalling frame bits alternate between three successive "0's" and three successive "1's" in odd-numbered frames. One of the basic functions of a PCM receiver is to determine which pulses in the pulse train are the main frame bits and to separate, based on that determination, the 24 channel segments from one another so that each segment can be fed into the proper channel after being decoded.
Circuitry employed for this purpose by the prior art is suitable but relatively complex. In addition, the speed at which frame information is recovered by prior art devices after an out-of-frame condition usually depends on the phase of the incoming signal relative to the receive timing circuitry, so that frame recovery can be quite slow under certain circumstances. In the highly competitive telephone equipment field, simplifications in circuitry which reduce costs while improving (or at least not significantly reducing) reliability or operational speed are often crucial from an economic point of view.
Another basic function of the receiver is to extract the signalling information from the incoming pulse train and to translate this information into the appropriate operational mode of the channel relays associated with each channel. In this respect also, prior art circuitry is operationally adequate but unnecessarily complex.